The possibility of women one day having children after the menopause appeared to come closer yesterday when US scientists reported that they had created an embryo from an egg produced from ovarian tissue that had been frozen for six years.

Storing and transplanting ovaries or ovarian tissue so that women who undergo a premature menopause after chemotherapy for cancer can have babies is one of the main goals of fertility researchers.

In an online paper, Kutluk Oktay and teams from the Centre for Reproductive Medicine and Infertility of New York Presbyterian hospital and Weill Cornell medical centre say that they unfroze some tissue taken from a 30-year-old woman before chemotherapy, and transplanted it under the skin of her abdomen.

After three months the tissue began to produce egg follicles. Twenty eggs were retrieved, of which eight were suitable for fertilisation with her husband's sperm. One successfully fertilised egg was allowed to develop to a four-cell-stage embryo and placed in the womb.

Although the woman did not become pregnant, the researchers say that getting that far was something of a breakthrough. "Prospects for a pregnancy and liveborn are now more promising," says the paper. In future, women who had cancer could "bank" an ovary before undergoing chemotherapy which could leave them infertile. If the research proves successful, the ovary would be frozen and stored until a patient decided she wanted to have a baby - at which point it would be thawed and reimplanted.

Hundreds of thousands of women worldwide are left infertile each year by cancer treatment - chemotherapy, radiotherapy or radical surgery. But embryologists warned that an effective treatment might be some time away, possibly beyond the lifetime of women who might seek to freeze an ovary now. Virginia Bolton, a clinical embryologist at King's College hospital in London, said the results were very encouraging.

"Up to now there has been very patchy help for women undergoing treatment that will render then sterile, very much depending on the oncologist involved.

"This technique is a big step forward - but we should not get overexcited as there is a long way to go before this becomes a routine treatment."

Dr Bolton, a member of the British Fertility Society, said women who asked for ovarian tissue to be frozen should be made aware that treatment may not become possible in their lifetime, so they are not given false hope.